The invention relates to an improvement to the powder material conveyance and distribution system in hyperdense phase. This improvement makes it possible to equip existing industrial installations with this high performance and economic conveyance system.
It is a continuous process for conveyance of a powder product in order to feed a large number of packaging assemblies such as bagging machines, containerization devices, or a large number of production assemblies such as plastic extruding presses or igneous electrolysis vat cells, from a single storage area.
Powder materials to be conveyed can be fluidized; their size grading and cohesion are such that injecting gas into them at low velocity can eliminate cohesion between particles and reduce internal friction forces. For example, this type of material includes alumina for igneous electrolysis, cements, plasters, quick lime or slaked lime, fly ash, calcium fluoride, magnesium chloride, all types of fillers for mixes, catalysts, coal dust, sodium sulfate, phosphates, polyphosphates or pyrophosphates, plastics in powder form, food products such as powder milk flour, etc.
Many devices have been studied and developed for conveyance of powder materials in fluidized bed. One particular problem is related to the continuous feed of the powder material regulated as a function of consumption requirements of the said material. One of the many examples illustrating this problem is feed of alumina to igneous electrolysis cells for the production of aluminum.
In order to do this, the alumina, which is a powder product conveyed and solubilized in the electrolytic bath, is consumed gradually while electrolysis is taking place, and must be replaced as it is consumed so that the concentration of solubilized alumina remains optimum, encouraging maximum efficiency of the electrolysis cell. It then becomes necessary to adjust the quantity of alumina added into the electrolysis vat, so that its operation is not disturbed by excess or insufficient alumina.
The powder materials conveyance device developed by the applicant and described in European patents EP-B-0 122 925, EP-B-0 179 055, EP-B-0 187 730, EP-B-0 190 082 and EP-B-0 493 279 enables continuous feed of powder solids in their hyperdense phase. It is used particularly for regular and continuous feed to storage and distribution hoppers located in the superstructure of electrolytic pots.
This device comprises at least one horizontal conveyor called the air-pipe between the storage area and the area to be supplied, composed of a lower duct in which gas circulates, and an upper duct in which the powder material is conveyed, the two channels being separated by a porous wall. Gas is blown into the lower duct through at least one supply tube. The powder material completely fills the upper duct of the conveyor and this conveyor is fitted with at least one balancing column partially filled with powder material, the filling height balancing the gas pressure. This balancing column creates the conditions for potential fluidization of the powder material. The powder material, which is not disturbed very much due to the very low gas flow, is present in the air pipe in the form of a hyperdense bed.
In order to make the description of potential fluidization easier to understand, it is worth while repeating the principles of conventional fluidization, normally used for conveying powder materials and described for example in patent U.S. Pat. No. 4,016,053. The device used in fluidization also comprises an air pipe as described above. The fluidization gas is injected into the lower duct at a given pressure pf, passes through the said porous wall and then passes between the particles at rest in the powder material forming the layer to be fluidized. Unlike the potential fluidization device, the thickness of this layer at rest is very much less than the height of the upper duct of the said conveyor, in other words in the absence of any injection of fluidization gas, the powder material only very partially fills the upper duct of the horizontal conveyor.
In conventional fluidization, by imposing a high gas flow, the said particles are moved and lifted, each of them losing its permanent contact points with its neighbors. In this way the interstitial space between the particles increases, internal friction between particles is reduced and these particles are put into a state of dynamic suspension. Consequently, the result is an increase in the initial volume of the powder material and a corresponding reduction in the apparent density.
The term xe2x80x9cdense phasexe2x80x9d is usually reserved for pneumatic transport at high pressure. The hyperdense phase is characteristic of potential fluidization. To give an idea of the situation, consider the example of the case of alumina Al2O3 in which the solid/gas ratio is of the order of 10 to 150 kg Al2O3/kg of air in dense phase pneumatic transport and is 750 to 950 kg Al2O3/kg of air for conveyance by potential fluidization in the hyperdense phase. Therefore, the solid powder can be conveyed at very high solid gas concentrations in the hyperdense phase, significantly higher than the dense phase in pneumatic transport.
In the case of potential fluidization, even if no gas is injected, the powder material almost completely fills the conveyance device and particularly the upper duct. When gas is injected into the lower duct, the balancing column is partially filled with powder material occupying the upper duct at a manometric head that balances the pressure pf and prevents the size of the interstices between the particles from increasing. Consequently, the presence of the balancing column prevents fluidization of the powder material present in the horizontal conveyor and forces the said material to appear as a hyperdense potential fluidization bed. Furthermore, since the interstitial distance between particles does not increase, the permeability of the medium to gas injected at pressure pf is very low and limits the gas flow to a very small quantity. We will subsequently refer to this low gas flow that passes through the balancing column xe2x80x9cdegassingxe2x80x9d.
Therefore, no fluidization takes place, but it is possible to talk about potential fluidization; there is no permanent circulation of material in the air pipe, but flow will take place by successive collapsing as soon as the need for any powder material arises, for example when the level of the area to be supplied drops below a critical value. When continuous consumption of the material stored in the area to be supplied is such that the material level drops below the level of the orifice in the supply pipe, a certain quantity of powder material will escape from the pipe creating a xe2x80x9cvacuumxe2x80x9d which will be filled by falling material, which will create a domino effect and thus continue throughout the air pipe working backwards towards the storage silo.
The potential fluidization device for conveyance in a hyperdense bed, as described in European patents EP-B-0 122 925, EP-B-0 179 055, EP-B-0 187 730, EP-B-0 190 082 and EP-B-0 493 279, is used on a large scale particularly to supply 300000 ampere vats in recent installations designed for igneous electrolysis of aluminum. The advantages of this device are well known:
continuous feed to vats in order to keep the hoppers full at all times,
low system maintenance, low wear due to the low product circulation velocity,
no size grading segregation,
low energy consumption,
perfect control over transport of the alumina, with no preferential blow off.
In an electrolysis workshop, the number of areas to be supplied from a single storage area may be high (several tens) and the distance between the storage area and the area to be supplied may be high (several hundreds of meters). The device illustrated in EP-B-0 179 055 is composed of a series of conveyors in cascade; a primary conveyor between the storage area and a series of secondary conveyors, each assigned to a pot and equipped with side take-off points that feed hoppers integrated into the vat superstructure.
But this system imposes the use of horizontal or slightly inclined conveyors, so that the sequence of small collapses (that occur progressively in the air pipe as far as the storage silo) can occur under optimum conditions. The applicant observed firstly that it is impossible to keep the material in a state of potential fluidization if the conveyor is inclined at a steep slope and secondly that a sudden change in the slope interrupts the xe2x80x9cdominoxe2x80x9d effect of small collapses and causes the formation of solid plugs in which the powder material can no longer be kept in the potential fluidization state.
However, an old workshop was not necessarily designed to be fed only by horizontal or slightly inclined conveyors. There are sometimes passageways inclined conveyors. There are sometimes passageways reserved for electrolysis service vehicles (liquid bath transport, metal transport, etc.) and obstacles that conveyors cannot bypass to the left or to the right, and where a level change is unavoidable.
Consequently, the applicant attempted to develop a process that would make it possible to use the hyperdense phase conveyance system described in European patents EP-B-0 122 925, EP-B-0 179 055, EP-B-0 187 730, EP-B-0 190 082 and EP-B-0 493 279, even for the purposes of renovating equipment in old installations.
The process according to the invention is a process for conveyance of powder materials by potential fluidization capable of bypassing obstacles by changing levels, in other words releasing the hyperdense bed conveyance system from the constraint of using only horizontal or slightly inclined conveyors. These conveyors are qualified as xe2x80x9chorizontalxe2x80x9d in the rest of this description, even if they are slightly inclined, for simplification purposes.
According to the invention, a device to bypass an obstacle comprising at least three caissons is inserted into the hyperdense bed conveyor system adjacent to the obstacle to be bypassed between two horizontal conveyors (one will be called the xe2x80x9cupstreamxe2x80x9d conveyor and the other the xe2x80x9cdownstreamxe2x80x9d conveyor):
at the entry to the device, an upstream caisson comprising a lower duct containing gas fed at pressure P1 and an upper duct or pipe composed essentially of a column connected at one end to the upper duct of the upstream conveyor and at the other end to the upper duct in the intermediate caisson;
in the middle, at a level that goes above the obstacle, at least one intermediate caisson comparable to a horizontal air pipe, the lower duct of which is fed with gas at pressure P3 and the upper duct of which is connected through its first end to the upper duct of the upstream caisson, is connected at its second end to the upper duct of the downstream caisson;
at the outlet from the device, a downstream caisson comprising a lower duct supplied with gas at pressure P2 and an upper duct or pipe composed essentially of a column connected firstly to the upper duct of the intermediate caisson and secondly to the upper duct of the downstream conveyor.
The obstacle is at the same level as at least one of the horizontal conveyors and the intermediate caisson is not at the same level as the obstacle, so that it can bypass it. The horizontal conveyors are usually at the same level, but there is no reason why there should not be a difference in height between these two horizontal conveyors. The intermediate caisson is long enough to get past the obstacle to the powder material to be conveyed.
The particular feature of the device is that it creates a pressure difference xcex94P=P1xe2x88x92P2 which is always strictly positive, the pressure difference being such that the three caissons remain full of powder material kept in a potential fluidization state at all times. By making sure that this pressure difference remains positive, the device acts like a hydraulic siphon; note that the product flow takes place freely, continuously and regularly from the first horizontal conveyor to the second.
Preferably, the columns on the upstream caisson and the downstream caisson are balancing columns full of powder product at a height such that the free level of the said material in each of these columns is at or above the highest point in the air pipes belonging to the group consisting of the intermediate caisson and the parts of the upstream and downstream conveyors located close to their junctions with the bypass device. Since the pressure difference xcex94P=P1xe2x88x92P2 is always positive and is guaranteed when the height of the powder material in the column of the upstream caisson is greater than the height of the powder material in the column of the downstream caisson.
In practice, the device according to the invention applied to the conveyance of alumina preferably uses an intermediate caisson located at a lower level than the two horizontal conveyors, so that alumina can go under the floor of the vats to leave free passage for electrolysis service vehicles. But a passage above or at an intermediate level would also be possible. The essential point is firstly to make sure that the free alumina level in the two columns is at the highest point of the two conveyors and the intermediate caisson, and secondly that the alumina height in the first column is greater than the alumina height in the second column.
The pressure in the intermediate caisson is used to put the powder material into a potential fluidization state. Preferably, its value is intermediate between the potential fluidization pressure of the first column and the potential fluidization pressure of the second column.
If the system is to operate correctly, it is useful to form a space without any powder material in the high part of the intermediate caisson upper duct, forming a pressurized gas bubble. The applicant has observed that in general, the presence of gas bubbles in the high part of upper ducts in hyperdense phase conveyors improves potential fluidization conditions and enables better circulation of the fluidization gas. The French patent application FR 9806124 deposited by the applicant on May 11, 1998 describes devices adapted to the creation of bubbles stable in the high part of the upper ducts of conveyors. In fact, it is sufficient to extend each column by a penetration on each side of the high part of the intermediate caisson upper duct. The height of the penetration is preferably between one half and one hundredth of the height of the useful part of the air pipe conveying the powder material.